1. Field of the Disclosed Embodiments
This disclosure relates to particular formulations for inks and particular methods for achieving those formulations for use in an emerging class of variable data digital lithographic image forming devices. Specific sub-system requirements that are unique to the emerging architecture caused an exploration of formula boundaries that, through experimentation, were determined to substantially reduce particle sizes for the color pigments enabling a higher latitude for ink image thickness. These ink formulations displayed improved rheological profiles allowing inks to have higher zero-shear or static viscosity making them less runny for use in the new class of variable data digital lithographic image forming devices.
2. Related Art
Lithographic and offset lithographic image forming are commonly understood printing methods for performing high quality multi-color images on a wide array of image receiving media substrates. For the purposes of this disclosure, the terms “printing,” “marking” and “image forming” may be used interchangeably. In a typical lithographic image forming process, an image transfer surface, which may be in a form of a flat plate, a surface of a cylinder or drum, a surface of a belt or the like is patterned to include “image regions” generally of hydrophobic/oleophilic materials, and “non-image regions” generally of hydrophilic/oleophobic materials. The image regions correspond to the areas on the final print of an image formed on a target image receiving media substrate that are occupied by a marking material, such as ink, to form the images on the target substrate. The non-image regions correspond to the areas on the final print that are not occupied by the marking material. The hydrophilic regions accept, and are generally readily wetted by surface preparation fluids, which may include water-based fluids or other compound fluids, which may be commonly referred to as dampening fluids or fountain solutions. In embodiments, these dampening fluids conventionally consist of water and small amounts of alcohol and/or other additives and/or surfactants that are included to reduce surface tension of the fluids.
The hydrophobic regions of, for example, a printing plate tend to repel dampening fluid and accept ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid “release layer” for rejecting the adherence of ink on the imaging surface of the printing plate. The hydrophilic regions of the printing plate thus correspond to unprinted, or “non-image,” areas of the final print.
In varying embodiments of conventional systems for lithographic image forming, the ink, as the marking material, may be transferred directly from the imaging surface to a target image receiving media substrate, such as paper or another substrate material at a pressure ink transfer nip. In offset lithographic image forming, the ink may be transferred from the imaging plate surface to an intermediate image transfer surface, such as an offset (or blanket) cylinder. Offset cylinders are often covered with conformable coatings or sleeves with surfaces that can conform to the texture of the imaging plate surface and the target image receiving media substrate, each of which may have, for example, a surface peak-to-valley depth somewhat different from the surface peak-to-valley depth of the other. Surface roughness or conformity of the offset (or blanket) cylinder helps to deliver a more uniform layer of the marking material, including ink, to the target image receiving media substrate free of defects such as mottle. Sufficient pressure is used to transfer the image directly from the imaging plate surface, or from the offset (or blanket) cylinder, to the target image receiving media substrate.
Typical lithographic and offset lithographic printing techniques employ plates that are permanently patterned, and are, therefore, useful for cost-effective image forming only when printing a large number of copies of the same image (i.e., for long print runs), such as magazines, newspapers, and the like. These techniques are not considered useful in creating and printing documents in which new patterns are generated from one page to the next without removing and replacing the print cylinder and/or the imaging plate. In this regard, conventional lithographic and offset lithographic printing techniques cannot accommodate true high-speed variable data printing in which the images may be changeable from impression to impression, for example, as in the case of what may be considered truly digital printing systems. Further, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is, therefore, higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems. Additionally, because images do not change from impression to impression, ink transfer efficiency from the imaging plate surface to one or the other of the offset cylinder or target image receiving media substrate can be comparatively imprecise. Typical of these conventional systems are ink formulations which transfer, on average, as little as 50% of the ink deposited on the imaging plate surface.